Runx3 plays a critical role in restriction-point and defense against cellular transformation.

The restriction (R)-point decision is fundamental to normal differentiation and the G1 -S transition, and the decision-making machinery is perturbed in nearly all cancer cells. The mechanisms underlying the cellular context-dependent R-point decision remain poorly understood. We found that the R-point was dysregulated in Runx3-/- mouse embryonic fibroblasts (MEFs), which formed tumors in nude mice. Ectopic expression of Runx3 restored the R-point and abolished the tumorigenicity of Runx3-/- MEFs and K-Ras-activated Runx3-/- MEFs (Runx3-/- ;K-RasG12D/+ ). During the R-point, Runx3 transiently formed a complex with pRb and Brd2 and induced Cdkn1a (p21Waf1/Cip1/Sdi1 ; p21), a key regulator of the R-point transition. Cyclin D-CDK4/6 promoted dissociation of the pRb-Runx3-Brd2 complex, thus turning off p21 expression. However, cells harboring oncogenic K-Ras maintained the pRb-Runx3-Brd2 complex and p21 expression even after introduction of Cyclin D1. Thus, Runx3 plays a critical role in R-point regulation and defense against cellular transformation.